Magnetic tape drives are used to store and retrieve information from magnetic tape. The information is recorded on and read from the tape as it moves over a magnetic read/write head. The magnetic “head” may be a single head or, as is common, a series of read/write head elements stacked individually and/or in pairs within the head unit. Data is recorded on tracks on the tape by moving the tape lengthwise past the head. The head elements are selectively activated by electric currents that represent the information to be recorded on the tape. The information is read from the tape by moving the tape longitudinally past the head elements so that magnetic flux patterns on the tape create electric signals in the head elements. These signals represent the information stored on the tape.
Data is recorded on and read from parallel tracks on the tape by positioning the head elements at different locations across the tape. Head elements are moved from track to track as necessary either to record or to read the desired information. An actuator operatively coupled to some type of servo control circuitry controls movement of the magnetic head. Tape drive head positioning actuators often include a lead screw driven by a stepper motor, a voice coil motor, or a combination of both. The head is positioned by the actuator along a path perpendicular to the direction that the tape travels.
Modern tape drives operate at high speeds and pass data to and from the tape where the data is stored. In order to maintain the precise degree of control required to operate in such an extreme environment, servo information recorded on the tape is employed to position the head elements as close as possible to the center of a track. A tape drive servo control system must receive accurate servo information from the tape in order to be able accurately to position the tape head. This requirement may have an inherent dilemma: the tape head must be positioned properly in order for the servo control system to be able to read servo information from the tape. At the same time, the servo control system must be able to read servo information from the tape in order properly to position the head. To break this cycle, a supplementary head positioning system often is used that places the head at a position close enough to the correct position that servo information can be read from the tape. The servo system then reads the servo information from the tape and finely adjusts the head position in order that data can be written to or read from the tape.
One example of a supplementary head positioning system employs a sensor for determining the mechanical position of the head. The sensor can include a variable inductor. A magnetic core secured to the actuator of the head positioning system moves with the actuator (and therefore with the head) in and out of a coil of wire. This motion changes the inductance of the coil. A processor measures the change in inductance and infers therefrom an estimate of the mechanical position of the tape head. The effect of hysteresis in magnetic cores leads to “backlash” when such a system is used to estimate mechanical position. Additionally, saturation and other nonlinear effects limit the accuracy with which heads can be positioned using magnetic methods. The variable inductance device also is quite expensive and so constitutes a less than totally satisfactory solution to the supplementary head positioning problem.